1. Field of the Invention
The present invention generally relates to spinal fixation systems and the like. More particularly, an embodiment of the invention relates to a spinal implant system for correction, fixation, and stabilization of the human spine to allow the development of a solid spinal fusion.
2. Description of the Related Art
Spinal fixation, such as lumbar sacral fusion and the correction of spinal deformities such as scoliotic curves, is a well known and frequently used medical procedure. Pedicle, lateral, and oblique mounting devices may be used to secure corrective spinal instrumentation to a portion of the spine that has been selected to be fused by arthrodesis.
A spinal fixation system typically includes corrective spinal instrumentation that is attached to selected vertebrae of the spine by screws, hooks, and clamps. The corrective spinal instrumentation includes spinal rods or plates that are generally parallel to the patient's back. The corrective spinal instrumentation may also include transverse connecting rods that extend between neighboring spinal rods. Spinal fixation systems are used to correct problems in the lumbar and thoracic portions of the spine, and are often installed posterior to the spine on opposite sides of the spinous process and adjacent to the transverse process.
Various types of screws, hooks, and clamps have been used for attaching corrective spinal instrumentation to selected portions of the patient's spine. Examples of pedicle screws and other types of attachments are illustrated in U.S. Pat. Nos. 4,763,644; 4,805,602; 4,887,596; 4,950,269; and 5,129,388. Each of these patents is incorporated by reference as if fully set forth herein.
An eyebolt assembly of the TSRH.RTM. spinal system sold by Danek Medical Inc. is illustrated in FIG. 1. The eyebolt assembly 2 encircles spinal rod 4 such that assembly mass completely surrounds the spinal rod. The spinal rod must be inserted through the eyebolt, which rests within the yoke of spinal hook 8. The spinal hook attaches the spinal rod to a bony element of the spine. A nut 6 is threaded onto a post of the eyebolt assembly to fixably secure the rod within the yoke. The nut is tightened so that the assembly resists axial, torsional, and shear forces to inhibit motion of the spinal rod relative to the assembly in the directions indicated by the arrows in FIG. 1. Further details of the TSRH.RTM. spinal system are provided in the TSRH.RTM. Spinal Implant System Surgical Technique Manual and the TSRH.RTM. Crosslink Surgical Technique Manual. Both of these publications are available from Danek Medical Inc. and are incorporated by reference as if fully set forth herein.
Manual insertion of a spinal rod through the bores of a number of spaced-apart eyebolts within a surgical wound tends to be difficult. The bore axis of each eyebolt must be properly aligned along a common axis, which is difficult since the corrective procedure requires that the spinal rod initially be placed under stress to resist deforming forces of the spine. Therefore, the use of systems such as the TSRH.RTM. spinal system may require that a predetermined number of screws or hooks be pre-loaded onto the spinal rod in a particular order and spacing prior to the insertion of the spinal rod into the surgical wound. After insertion of the spinal system into the surgical wound, however, it is often necessary to add, delete, or reposition one or more hooks or screws. Before such modifications can be made, the spinal system typically must be removed from the surgical wound and at least partially disassembled.
To overcome such problems, some spinal fixation systems include "open back" hooks or screws to allow a spinal rod to be dropped into the open back of the hook or screw and secured within the open back by a separate component and a set screw. Such a system is illustrated in U.S. Pat. No. 5,102,412 to Rogozinski, which is incorporated by reference as if fully set forth herein. Such systems tend to be susceptible to fatigue stress failure and require assembly within the surgical wound. In addition, adding a hook or screw to the construct tends to require that the spinal rod first be repositioned. A further disadvantage of this approach is that component mass completely surrounds the spinal rod, resulting in an increase in the profile width of the device and greater impingement of the device upon the fusion mass. A low profile width is generally desired to minimize sinus formation and soft tissue irritation from hardware prominence.
U.S. Pat. No. 5,242,445 to Ashman relates to a "split eyebolt" assembly for adding eyebolts to an assembled spinal fixation construction. Attaching the split eyebolt to a spinal rod requires a special crimping tool to crimp the split eyebolt over the rod. The crimping tool tends to be difficult to operate within the surgical wound. Furthermore, the threads of the opposing sides of the split eyebolt are often misaligned after crimping, making it difficult or impossible to thread a nut onto the split eyebolt. The split eyebolt also completely encircles the spinal rod thereby increasing the impingement of the construct upon the fusion mass.
It is therefore desirable that an improved spinal fixation system be derived that facilitates assembly and surgical implantation by allowing the spinal rod to be positioned within the surgical wound (a) after the fixation components (e.g., screws, hooks) have been implanted, (b) without modifying the fixation components, and (c) whereby fixation components may be subsequently added, deleted, and/or repositioned without disassembling the system.